This invention relates to optical fiber couplers for division and junction of light in optical communication, optical fiber sensors, etc., more particularly to polarization-maintaining optical fiber couplers which enables division (bifurcation) of junction (coupling) of light with maintaining the direction of polarization of light.
Conventional examples of optical fiber couplers which can be used for the above purpose include polished optical fiber couplers as shown in FIGS. 1, 2, 3a and 3b, in which reference numeral 1 designates an optical fiber coupler, 2a and 2b polished optical fibers. The optical fiber coupler 1 comprises polished optical fibers 2a and 2b and two prismatic glass blocks or pillars 2a and 3b to which portions of the polished optical fibers are fixed, the glass blocks 3a and 3b being bonded or connected to each other with bringing the polished portions 4a and 4b of the polished optical fibers 2a and 2b in contact with each other. The glass blocks 3a and 3b are each provided with a groove 5 along the longitudinal axis thereof in which the polished optical fiber 2a or 2b is held or embedded. The bottom of the groove is shaped in an arcuate form or slightly convexed so that the polished optical fibers embedded therein can be given a suitable bending as shown in FIG. 2. The polished optical fibers 2a and 2b have polished portion or area 4a and 4b, respectively, which are brought into contact with each other at a contact portion 6 where cores 7a and 7b, respectively, of the polished optical fibers 2a and 2b are positioned close to each other so that light power transmitted in the core of one polished optical fiber (e.g., the core 7a of the optical fiber 2a) leaks to the outside and enters the core of another polished optical fiber (the core 7b of the optical fiber 2b) to achieve junction or coupling of light power. That is, incident light power which entered in an incident port of the optical fiber 2a is divided at the contact portion 6 and a part thereof is transmitted in the other polished optical fiber 2b, and divided or bifurcated light powers at a predetermined output bifurcation ratio are obtained from an output port 9 of the polished optical fiber 2a and a divisional or branch output side port 10 of the polished optical fiber 2b.
The optical fiber coupler 1 is manufactured in fabrication steps as shown in FIGS. 4 to 7. That is, at first, a groove 5 for holding or embedding an optical fiber is formed on a rectangular parallelepiped substrate 3 made of a transparent material such as quarts based glass, multi-component glass, fluoride glass, etc. The groove 5 is formed by grinding the rectangular parallelepiped substrate 3 on one of surfaces having the largest area (b.times.c) (FIG. 4), say surface A, along the length c or in the longitudinal direction with respect to the substrate 3. The cross-section of the groove 5 is U-shaped in FIG. 4 but may be in an other form, e.g., square or rectangular.
In the above operation, a grooved substrate or glass block 3 as shown in FIG. 4 is prepared. Then, as shown in FIG. 5, an optical fiber 2 is inserted or placed in the groove 5 of the grooved substrate 3. The optical fiber 2 comprises a core 7 and a cladding 11 surrounding the core 7. In the case of stress-applying polarization-maintaining optical fibers, the optical fiber 2 further comprises two stress-applying parts 12, 12 arranged so as to sandwich the core 7 therebetween. In the following, description will be made with respect to stress-applying polarization-maintaining optical fibers (hereinafter, sometimes referred to as "optical fibers" or "fibers" for simplicity), however, the same explanation is valid for non-stress-applying polarization-maintaining optical fibers.
When inserting a polarization-maintaining optical fiber in the groove 5 it is important to align the polarization principal axis of the fiber in the direction of the depth of the groove. There are two polarization principal axes in the core of each stress-applying polarization-maintaining optical fiber. Generally, the axis connecting the centers of the stress-applying parts to each other is called "X-axis" and the axis at right angles to X-axis "X-axis". In the case shown in FIG. 3b, Y-axis is aligned in the direction of the depth of the groove 5. To align the polarization principal axis with the direction of the depth of the groove, that is, the direction of polishing is very important in the manufacture or fabrication of polarization-maintaining optical fibers. Disagreement for misalignment of the polarization principal axis between the polished optical fibers in a polarization-maintaining optical fiber will result in the deterioration of polarization maintenance characteristics of the fiber.
Optical fibers are fabricated by pouring or filling an adhesive 13 into the groove 5 followed by allowing the adhesive to cure. By this, the optical fiber 2 is fixed to the glass block 3 (FIG. 6). The amount of light power which leaks depends on the amount or degree of polishing at the polished part 4, and various types of glass blocks 3a, 3b with different amounts of polishing are fabricated depending on the purposes for which the fiber couplers are used. Then, the two glass blocks 3a, 3b are bonded or connected to each other so that polished parts 14a and 14b of the glass blocks 3a and 3b, respectively, are in contact with each other through an index matching oil. The relative position of one glass block with respect to another, i.e., the position of the glass block 3a with respect to the glass block 3b is adjusted by monitoring light power emitted from a light source 16 using light detectors 17 and 17 connected to output port 9 and branch output port 10, respectively, and translating one of the glass blocks or both of them so as to form relative positional change therebetween until a position is reached where a predetermined output bifurcation ratio is exactly obtained (FIG. 1). Then, the glass blocks 3a and 3b are fixed to each other with an adhesive, etc. to form an optical fiber coupler 1. Further, there have heretofore been provided fused optical fiber couplers obtained by fusing under heating and drawing two polarization-maintaining optical fibers polarization-maintaining optical fibers which allows junction of light power at the fused parts. However, with this type of conventional optical fibers no high performance optical fiber coupler that gives accurate output split ratio can be prepared with ease. More particularly, upon fabrication of optical fiber couplers by melt-fusion, it is necessary to very precisely adjust the position of the core of each fiber so that the completed optical fiber coupler can have a predetermined output bifurcation ratio. However, the above-described conventional fiber couplers are so constructed that a part of each optical fiber is melt-fused and elongated to render the core of one optical fiber close to that of another, with the adjustment of the position of the cores being carried out depending on the degree of elongation, resulting in that it is difficult to perform accurate adjustment of the cores upon fusing.
It is also necessary to accurately control the amount of polishing at polished parts 4, 4 in optical fibers fabricated by polishing in order to obtain a desired output bifurcation ratio. For example, in order to render the output bifurcation ratio smaller at the branch output port 10 (cross path) and larger at the output side port 9 (straight path), optical fiber couplers must be fabricated using polished optical fibers 2a and 2b whose polishing amount at the polished parts 4, 4 is small thus making the distance L between the surface of the polished part 4 and the center of the core (FIGS. 3a and 3b) small. On the other hand, when it is intended to obtain fiber couplers with its bifurcation ratio being set up such that the ratio is larger on the branch output port 10 than on the straight output port 9 polished optical fibers 2a and 2b must be those with a large amount of polishing so as to make the distance L small.
For the above reason, the conventional manufacturing process for the fabrication of optical fiber couplers 1 is very complicated and the efficiency thereof is low. Since the amount or degree of polishing must be accurately controlled sometimes there occurs many unacceptable products with too much polishing, thus decreasing the percentage of acceptable products.
In order for optical fiber couplers having the above construction to have desirable characteristics such as accuracy in coupling ratio of light lower, low coupling loss of light power, etc. the surface A of the substrate 3 on the side where the groove 5 is provided must have very delicate flatness. As to the surface A, it is necessary to accurately control not only its flatness but also the amount of polishing, i.e., the distance L between the surface A and the core 7. It has heretofore been difficult to polish the substrate in such a manner that both the above requirements are met since various factors adversely affect the quality of polished surface. For example, minute protrusion and depression in a polishing plate tend to cause unevenness in the surface A and the polished part 14 as shown in FIGS. 8 and 9. The polished part 14 with uneven surface is undesirable since an index matching oil 15 which intervenes between the surfaces A is localized, thus giving various adverse influences on the resulting optical fiber coupler such as those on the refractive index from the optical viewpoint and local concentration of stress due to expansion and contraction on the polished part 14 from mechanical viewpoint, resulting in drastic change in the coupling characteristics of the optical fiber coupler due to temperature change or other factors, which in turn deteriorates the antiweatherability or environment resistance of the coupler.
The degree of the deterioration of the environment resistance is substantially proportional to the area of the uneven surface A being in contact. The larger the area the more deteriorated the environment resistance. The magnitude of unevenness or protrusion and depression in the polished part does not give a serious influence on the environment resistance when it is smaller than about half the wavelength of light used (about 0.5 micrometers) but greatly deteriorates environment resistance when it is larger than the size in the order of about the wavelength (about 1 micrometer). On the other hand, it is the more advantageous the larger the area of another surface of the substrate which is in the side opposite to the surface A (hereinafter, referred to as "surface B") since stability upon holding the substrate by a holding jig (not shown) and thus accuracy or precision of polishing are improved.
Further disadvantage of the conventional polishing process is that as shown in FIGS. 6 and 7, the direction 27 of polishing is perpendicular to polarization principal axis 26 and as the result it has been difficult to exactly align polarization principal axis Y in the direction of polishing since usually alignment of polarization principal axes is conducted by observing or detecting the position of the stress-applying part under microscope in the direction of polishing and in this case it is difficult to perform precise alignment because of the stress-applying parts being arranged at right angles to the direction of the observation as shown in FIG. 6. Accordingly, when polarization-maintaining optical fiber couplers are fabricated using the polished optical fibers which were prepared in the manner as shown in FIGS. 6 and 7, the resulting coupler hardly have acceptable polarization maintenance characteristics. Another serious problem is that cracks 28 occur in the stress-applying parts 12. There is always a contractile force directed toward the center in the stress-applying part or zone. Therefore, the stress-applying part is susceptible to shoes from outside and suffer a high possibility of the occurrence of cracks 28 upon polishing or lapping. Once cracks 28 occur the polarization maintenance characteristics of the coupler is greatly deteriorated. What is worse, polishing both of the two stress-applying parts makes the stress-applying effect on the core 8 is greatly reduced.